Wireless communication devices having multiple antennas arranged in a diversity configuration offer a variety of transmission and reception benefits compared to devices with just a single antenna. The basis of diversity is that, at any given time, the antenna with the best reception is selected for reception or transmission. Although a device utilizing antenna diversity may have multiple physical antennas, there is only a single set of electronic circuitry to process the signal, also called a radio frequency (RF) chain.
Multiple in-multiple out (MIMO) wireless technology improves upon antenna diversity by utilizing multiple RF chains. Each RF chain is capable of simultaneous reception or transmission. This allows a MIMO device to achieve higher throughput and to resolve negative effects of multipath interference. In a transmitting device, each RF chain is responsible for transmitting a spatial stream. A single frame can be disassembled and multiplexed across multiple spatial streams, which are then reassembled at a receiver.
MIMO is one of the most promising techniques in wireless communications. Unlike traditional smart antenna techniques that aim to mitigate detrimental multipath fading and enhance robustness of a single data stream, MIMO takes advantage of multipath fading to transmit and receive multiple data streams simultaneously. Theoretically, the capacity in a MIMO system increases linearly with the number of transmit and receive antennas. MIMO is being considered by numerous wireless data communication standards, such as IEEE 802.11n and 3GPP wideband code division multiple access (WCDMA).
In implementing MIMO, a WTRU may operate in either a spatial multiplexing mode or a spatial diversity mode. In the spatial multiplexing mode, a WTRU transmits multiple independent data streams to maximize data throughput. While in the spatial diversity mode, a WTRU may transmit a single data stream via multiple antennas. Depending on the operation mode, a WTRU is configured to select an appropriate quality metric or a combination of quality metrics to utilize in the selection of a desired beam combination. Typically, an m×N channel matrix H is obtained of the form:
      H    =          [                                                  h              Aa                                            ⋯                                              h              Na                                                            ⋯                                                                                          ⋯                                                              h              Am                                            ⋯                                              h              Nm                                          ]        ,where the subscripts of the elements h represent contributions attributable to each antenna mapping between transmitting WTRU A's antennas a . . . m and a receiving WTRU N's antennas a . . . m.
A WTRU may obtain a calibration matrix (K) in a similar manner. Calibration in the context of wireless LANs involves calculating a set of complex-valued correction coefficients that, when multiplied at the transmitting WTRU's baseband streams on a per-antenna and per-sub-carrier basis, would equalize the response difference between transmit and receive processing paths (up to an unknown constant across antennas).
Referring to FIG. 1, a signal diagram 100 of prior art channel calibration is shown. A transmitting WTRU (Tx WTRU) 110 first needs to calibrate the existing channel between receiving WTRU (Rx WTRU) 120. Tx WTRU 110 transmits a calibration training frame (CTF) 131 to Rx WTRU 120. Rx WTRU 120 responds by transmitting a sounding physical packet data unit (PPDU) 132. Tx WTRU1 110 calculates a channel estimation H 133 for the channel, referred to as H(2→1). Tx WTRU 110 transmits a calibration response 134 which includes the channel estimation H(2→1). Rx WTRU 120 then performs channel estimation by transmitting a CTF 135 to Tx WTRU 110. In response, Tx WTRU 110 transmits a sounding PPDU 136. Rx WTRU 120 calculates a channel estimation H(1→2), and calculates a calibration matrices K(1→2) and K(2→1) for the channel 137. Rx WTRU 120 then transmits a calibration response 138 including calibration matrix K(1→2) to Tx WTRU 110. It should be noted that the calibration matrix K(1→2) is then applied at Tx WTRU1 110 as a baseband gain or phase correction factor for transmissions to Rx WTRU 120. The calibration matrix K(2→1) is applied at Rx WTRU 120, again as a baseband gain/phase correction factor, in Rx WTRU's 120 transmission of signals to Tx WTRU 110. The channel is now calibrated and ready for packet exchange.
To initiate data packet exchange, Tx TWRU 110 transmits a request 139 to the Rx WTRU 120, which responds by sending modulation and coding scheme (MCS) PPDU 140. Tx WTRU 110 uses the calibration matrix K(1→2) to calculate a steering matrix V, and packet data transfer 142 begins.
The prior art does not consider the utilization of smart antenna technology. Smart antennas, and beamforming in particular, is a signal processing technique used with arrays of transmitters or receivers that controls the directionality of, or sensitivity to, a radiation pattern. When receiving a signal, beamforming can increase the gain in the direction of wanted signals and decrease the gain in the direction of interference and noise. When transmitting a signal, beamforming can increase the gain in the direction the signal is to be sent. When beamforming capable antennas are combined with MIMO, the number of available antenna mappings dramatically increases.
When beamforming antennas are included in a WTRU, the number of available antenna mappings may become very large. In order to optimize the communication link between two WTRUs, it is necessary to select the appropriate antenna mapping at both the transmitter and the receiver.
Therefore, a method and apparatus for efficiently utilizing the variety of available antenna mappings in a MIMO capable wireless device having multiple beamforming antennas is desired.